1. Field of the Invention
The present invention relates to a magnetic memory device, such as the arrangement of a memory cell and a write line.
2. Description of the Related Art
A magnetic random-access memory (MRAM) has a feature of having a memory cell of a magnetoresistive element that uses the magnetoresistive effect to store data and use a current to perform a write operation.
A magnetoresistive element is an element that uses the tunneling magnetoresistive effect, generally has a structure wherein one insulating layer is sandwiched between two ferromagnetic layers. One of the ferromagnetic layers is called a reference layer whose magnetization direction is fixed. The other one is called a recording layer whose magnetization direction is not fixed. The tunneling magnetoresistive effect is a phenomenon wherein a tunneling current changes when the magnetization direction of two ferromagnetic layers are parallel and anti-parallel.
By assigning a corresponding parallelism or anti-parallelism of the magnetization direction of the two ferromagnetic layers to “0” data and “1” data, the magnetoresistive element can retain 1 bit of data. A write operation is implemented by changing parallelism or anti-parallelism of a magnetization direction of the recording layer with the reference layer by a magnetic field generated by a current.
In order to realize a large-scale MRAM, the following three problems need to be dealt with: 1) to prevent a current flowing in a write line for writing to a magnetoresistive element of a write target from erroneously writing to a magnetoresistive element adjacent the target element, 2) to retain a high degree of thermal disturbance resistance of a magnetoresistive element, 3) to keep a current low.
As a countermeasure to the problems, a so-called toggle-writing is proposed (e.g., U.S. Pat. No. 6,545,906B1). Toggle writing uses two antiferromagnetically coupled ferromagnetic layers as ferromagnetic layers for recording data. Furthermore, the way it applies a magnetic field at the time of writing data is different from the above general way of writing. Toggle writing improves resistance against erroneous writing to an adjacent magnetoresistive element. However, the intensity of the magnetic field to switch the data retained by a magnetoresistive element is considerably larger than for a single layer recording layer. Therefore, along with miniaturization, the current for writing is expected to be over acceptable level. More specifically, for example, in a generation in which the length of the short side of a magnetoresistive element is less than 50 nm, realization of MRAM is very difficult to achieve.